1. Field of the Invention
The present invention relates to a valve gear with a cylinder suspending mechanism of an internal combustion engine (hereinafter referred to as engine).
2. Description of the Related Art
To realize the most suitable engine output characteristics according to an operation region, various types of engines, which perform the switching of the opening period or the lift amount of intake and exhaust valves, and the like, have been suggested (for instance, refer to Unexamined Japanese Patent Publication Nos. 2001-41017 and 2002-227624).
In the engine disclosed in Publication No. 2001-41017, a driven rocker arm supported by a rocker shaft is oscillated by a first cam to drive an intake valve to be open or close, and a drive rocker arm formed adjacent to the driven rocker arm and supported by the rocker shaft is oscillated by a second cam. A piston is fitted into a cylinder formed in the driven rocker arm so as to be slidable by hydraulic pressure, while in the drive rocker arm, an engaging projection that is engageable with the piston along with the oscillation is formed.
In a low revolution region of the engine, for instance, the piston of the driven rocker arm is switched to its lower position to cause the engaging projection of the drive rocker arm to strike at the air, to thereby open and close the intake valve along the shape of the first cam by using the driven rocker arm. In a high revolution area of the engine, on the contrary, the piston of the driven rocker arm is switched to its upper position to be pressed by the engaging projection of the drive rocker arm to oscillate the driven rocker arm with the drive rocker arm, to thereby open and close the intake valve along the shape of the second cam.
In the engine disclosed in Publication No. 2002-227624, a first rocker arm corresponding to a low-speed cam, a second rocker arm corresponding to a high-speed cam, and a third rocker arm corresponding to a suspension cam are supported in an intake-side rocker shaft, and a pair of intake valves is driven to open or close in liaison with the first and third rocker arms. There is provided a switch pin in each rocker arm to be slidable by hydraulic pressure in the axial direction of the rocker shaft. The rocker arms are connected with or disconnected from one another according to the sliding position of the switch pins.
In the low revolution region of the engine, for example, the connections of the rocker arms are released by the switch pins. One of the intake valves is driven to be open or close along the low-speed cam through the first rocker arm, and the other intake valve is practically kept closed by the suspension cam through the third rocker arm. In the high revolution region of the engine, the rocker arms are connected by the switch pins, and all the rocker arms are integrally oscillated by the high-speed cam to drive both the intake valves to be open or close along the high-speed cam.
As described above, in the engine disclosed in Publication No. 2001-41017, the switching is carried out in two operation states in each of which the intake valve is driven to be open or close by means of a corresponding one of the first and second cams. In the engine disclosed in Publication No. 2002-227624, the switching is performed in two operation states in one of which the intake valves are driven to be open or close by the low-speed cam and the suspension cam and in the other of which the intake valves are driven to be open or close by the high-speed cam. It has been desired to provide a cylinder suspending function for suspending certain cylinders in addition to the above-stated functions.
To add the cylinder suspending function, however, it is required to add a mechanism for stopping the oscillation of the rocker arms connected to the intake valves (namely the driven rocker arm in Publication No. 2001-41017 and the first and third rocker arms in Publication No. 2002-227624). This generates a problem that, with technologies disclosed in Publication Nos. 2001-41017 and 2002-227624, the entire valve gear would be enlarged in size as the configuration thereof becomes complicated.
A valve gear with a cylinder suspending mechanism requires that intake and exhaust valves of suspending cylinders be kept closed. For instance, as for an SOHC-type engine in which intake and exhaust valves are driven to open or close by a single camshaft, a circular suspension cam is provided on the camshaft, and intake and exhaust rocker arms are brought into contact with the suspension cam when cylinders are inoperative, to thereby hold the intake and exhaust valves closed. It is required in such an engine to form as many suspension cams as rocker arms on the camshaft. This causes problems, however, that the camshaft has to be long in its entire length, that the manhours of processing the camshaft are increased, and so on. Therefore, countermeasures against these problems have been suggested (refer to Unexamined Japanese Patent Publication No. 2002-201921 (FIG. 2)).
Publication No. 2002-201921 discloses an engine in which intake-side and exhaust-side rocker shafts each support a pair of drive rocker arms corresponding to a pair of intake and exhaust valves. According to the sliding movement of switch pins, the drive rocker arms are connected with or disconnected from their respective free rocker arms that are each oscillated by an intake cam, real suspension cam, and exhaust cam of a camshaft, to thereby arbitrarily open and close the intake and exhaust valves. The intake-side and exhaust-side drive rocker arms are located such that the intake-side drive rocker arms substantially correspond to their respective exhaust-side drive rocker arms in the axial direction of the rocker shaft, and a pair of suspension cams is formed on the camshaft so as to be located in between the corresponding intake and exhaust drive rocker arms. At the time of cylinder suspension, the corresponding intake and exhaust drive rocker arms are brought into contact with the common suspension cam, to thereby keep the intake and exhaust valves closed.
In an engine disclosed in Unexamined Japanese Patent Publication No. 2002-201921, suspension cams are reduced in number by half by providing a common suspension cam to intake and exhaust drive rocker arms. On the other hand, however, a pair of suspension cams is indispensable per cylinder due to constraints on layout of a cylinder suspending mechanism.
More specifically, as illustrated in FIG. 2 of the publication, a free rocker arm oscillated by an intake cam is disposed in between a pair of intake-side drive rocker arms, and another free rocker arm oscillated by an exhaust cam is arranged in between a pair of exhaust-side drive rocker arms. Such layout makes it impossible to substitute a single suspension cam for a pair of suspension cams, that is, to bring all the intake and exhaust drive rocker arms into contact with the single suspension cam, since the free rocker arms and the intake and exhaust cams cause obstruction.
Accordingly, the engine disclosed in the publication requires that a pair of suspension cams be formed on the camshaft per cylinder, which discourages full achievement of reduction of a camshaft's entire length and a reduction in manhours of processing the camshaft.